Method for reclaiming constituents from an industrial waste stream

ABSTRACT

An industrial waste stream recycling method for recovery of high purity zinc oxide products and other chemical and metal values from industrial waste streams containing zinc compounds by leaching the waste stream with a solution of 30% or greater by weight ammonium chloride, resulting in a first product solution and undissolved materials; adding zinc metal to the first product solution, whereby zinc-displaceable metal ions contained in the first product solution are displaced by the zinc metal and precipitate out of the first product solution as metals, leaving a second product solution; and diluting the second product solution with water, resulting in the precipitation of zinc oxide.

STATEMENT OF RELATED APPLICATIONS

This application is a continuation-in-part of Application Ser. No.09/338,129, filed on Jun. 22, 1999, currently pending U.S. Pat. No.6,264,903.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to a process for the reclamation ofconstituents contained in common industrial waste streams includingessentially pure metal oxides and metals. This invention morespecifically relates to a method for the recovery of essentially purezinc oxide and other compounds such as lead compounds, copper compounds,silver compounds, and cadmium compounds from electric arc furnace (EAF)dust emanating from the steel industries processes.

2. Description of Related Art

Metallurgical processes, such as steel processes, result in wasteby-products of iron and steel dust. There have been many attempts torecover valuable metal and chemical values such as zinc, lead, cadmium,silver, and copper in this dust and to obtain a by-product or finalwaste material that can be recycled or disposed of with minimal metalleaching problems.

Zinc oxide typically is a fine white or grayish powder that has avariety of uses including as a rubber accelerator, as a pigment, as adietary supplement and in the semiconductor field. Zinc oxide is foundin commercial by-products including waste material streams such as flyash and flue dust. Methods for recovering zinc oxides are known in theart, including recovering zinc oxide from industrial waste materials.Such previous methods have included leaching with mineral acid, causticsoda, ammonium hydroxide, and ammonium carbonate solutions. However,these methods have low yields of zinc oxide and typically do not recoverpure zinc oxide, the recovered zinc oxide being contaminated with othermetal salts. Therefore, in order to obtain pure zinc oxide, subsequentreduction and washing processes were necessary.

One such method for recovering zinc and lead is the Waelz kiln. TheWaelz kiln uses a directly heated counterflow rotary kiln to roast thematerials under reducing conditions. The reducing roasting processcomprises directly heating and roasting the iron and steel dust in theWaelz kiln in a reducing atmosphere under suitably selected conditionsof temperature and retention time, thereby separating zinc and leadthrough volatilization from the dust and enabling iron to be dischargedin the form of solid direct reduced iron. U.S. Pat. No. 4,525,208 toYasukawa attempts to avoid perceived problems with the Waelz kiln causedby the depositing of material on the walls of the rotary kiln by runningthe volatilization in two stages. In the first stage the material isheated and zinc and lead are partially evaporated at a lower temperaturein a rotary kiln. In the second stage, the solid material from therotary kiln is continuously fed into the rotary smelting furnace wherefluxes are added to the material to lower the melting point facilitatingthe evaporation of the metals from the molten stage.

U.S. Pat. No. 3,850,613 to Allen discloses a method for the treatment ofsteel mill waste dusts containing zinc. The Allen '613 method includesbriquetting the dust with carbon to reduce the zinc and lead oxides tothe zinc and lead metals and then volatizing the zinc and lead at hightemperatures by heating the briquettes to 1800° F. to 2500° F., and thenoxidized to ZnO and PbO in the gaseous phase. The ZnO and PbO then arerecovered, with the remainder of the material used as the charge to asteel-making furnace.

U.S. Pat. No. 5,013,532 to Sresty discloses a method for recyclingelectric arc furnace dust. The Sresty '532 method for reducing the zinccontained in an EAF dust, volatilizing the metallic zinc so producedfrom the mass of the dust, and reoxidizing the metallic zinc to zincoxide along with the simultaneous regeneration of hydrogen that can berecycled to treat additional EAF dust. The Sresty '532 process involvesheating a raw material containing a mixture of metals and metal oxidesin briquette, pellet, granular or lump form in a furnace to about 900°C. to about 1200° C., contracting the heated raw materials with a streamof hydrogen gas to reduce and vaporize the desired metal oxide oroxides, humidifying the stream of hydrogen gas and metallic vaporsrecovered from the furnace with water in the form of water vapor orsteam to lower the temperature of the gas stream to about 700° C. toabout 900° C. so that the metallic vapors react with the water toproduce a solid metallic oxide or mixture of metallic oxides,humidifying again to a temperature in the range of about 150° C. toabout 250° C., separating the solidified metal oxide from the hydrogengas stream, condensing excess water from the hydrogen gas stream, andrecycling the hydrogen gas stream.

U.S. Pat. No. 3,262,771 to Ban discloses a system for the recovery ofsteel and zinc from waste materials using a molten stage, including anelectric arc furnace, an electrothermic smelting furnace and a slagfuming method. In the Ban '771 process, the recovery of zinc and lead iscarried out in a molten stage by using an electric arc furnace. Zincoxide is recovered as an overhead vapor, and iron is recovered as aliquid.

U.S. Pat. No. 5,188,658 Aune also discloses a method for the recovery ofzinc from zinc-containing waste materials using a molten stage,including an electric arc furnace, an electrothermic smelting furnaceand a slag fuming method. In the Aune '658 process, zinc and lead arerecovered from the molten stage in an electrothermic smelting furnace.The electrothermic smelting furnace described in the Aune '658 patentrequires that the furnace be kept at high temperatures in order tomaintain a volume ratio between CO₂ and CO in the gas atmosphere in thesmelting furnaces below 0.3.

U.S. Pat. No. 3,017,261 to Lumsden also discloses a method for therecovery of zinc from zinc-containing waste materials using a moltenstage, including an electric arc furnace, an electrothermic smeltingfurnace and a slag fuming method. In the Lumsden '261 patent, zinc andlead are recovered from the molten stage with a slag fuming method usinga stationary furnace where metals are volatilized by melting the ironand steel dust completely and blowing air and reducing agent such ascoal or coke into the molten iron.

U.S. Pat. No. 4,840,671 to Lynn discloses a process to render suchindustrial waste non-hazardous without recovering zinc or lead. In theLynn '671 process, electric arc furnace dust, steel dust from theproduction of certain specialty grades of steel, is rendered lesshazardous by complexing the dust in a lime kiln dust, fly ash andhydrated lime mixture and then adding an aqueous solution containingferrous hydroxide and calcium sulfate. The Lynn '671 process is based onthe pozzolanic reaction of materials containing anhydrousalumino-silicates that, in the presence of lime, water and chemicals,adsorb and/or physically entrap the heavy metals present in EAF dustinto a calcium-alumino-silicate matrix.

U.S. Pat. No. 3,849,121 to Burrows discloses a method for the selectiverecovery of zinc oxide from industrial waste. The Burrows '121 methodcomprises leaching a waste material with an ammonium chloride solutionat elevated temperatures, separating iron from solution, treating thesolution with zinc metal and cooling the solution to precipitate zincoxide. The Burrows '121 patent discloses a method to take EAF dust thatis mainly a mixture of iron and zinc oxides and, in a series of steps,to separate out the iron oxides and waste metals. However, the materialobtained in the last step is a mixture of a small amount of zinc oxide,hydrated zinc phases that can include hydrates of zinc oxide and zinchydroxide, as well as other phases and a large amount of diamino zincdichloride Zn(NH₃)₂Cl₂ or other similar compounds containing zinc andchlorine ions. Currently, the Burrows '121 method is not economicallyviable because of Environmental Protection Agency guidelines establishedsubsequent to the issuance of the Burrows patent. Additionally, theBurrows '121 method is not a continuous method and, therefore, is noteconomical as a continuous process.

U.S. Pat. No. 4,071,357 to Peters discloses a method for recoveringmetal values which includes a steam distillation step and a calciningstep to precipitate zinc carbonate and to convert the zinc carbonate tozinc oxide, respectively. Peters '357 further discloses the use of asolution containing approximately equal amounts of ammonia and carbondioxide to leach the flue dust at room temperature, resulting in theextraction of only about half of the zinc in the dust, almost 7% of theiron, less than 5% of the lead, and less than half of the cadmium. Steamdistillation is contrary to dilution. Steam distillation precipitateszinc carbonate, other carbonates and iron impurities. Steam distillationalso disadvantageously results in an increase in temperature that drivesoff ammonia and carbon dioxide, resulting in the precipitation of ironimpurities and then zinc carbonate and other dissolved metals. Thepurity of the zinc carbonate obtained depends on the rate of steamdistillation and the efficiency of solids separation as a function oftime. Calcining converts the zinc carbonate to zinc oxide, whereaswashing and drying at temperatures between 100° C. and 200° C. convertsthe zinc compounds to zinc oxide.

U.S. Pat. No. 5,464,596 to Myerson, commonly assigned with the presentapplication, discloses a method for the recovery of zinc oxide bytreating a waste stream with a 23% ammonium chloride at 90° C.,separating undissolved components from the solution, displacingundesired metal ions from the solution using zinc metal, cooling thesolution to precipitate out zinc compounds, washing the precipitate toremove various soluble zinc compounds, leaving zinc oxide of greaterthan 99%. The Myerson '596 patent teaches that ammonium chloridesolutions must be at least 90° C. to sufficiently dissolve the zinccompounds. Heating an aqueous solution to such a temperature requiresthe expenditure of large amounts of energy. It is further taught thatwhile NH₄Cl concentrations below 23% do not dissolve the maximum amountof zinc oxide from the waste material, concentrations greater than 23%result in an impure zinc oxide due to the tendency of the NH₄Cl toprecipitate out of solution with the zinc compounds at such highconcentrations. Furthermore, the cooling of the product solution resultsin the precipitation of various zinc species, resulting incrystallization of some species. Because of this, using the cooling stepdisclosed in Myerson '596, one cannot use concentrations of ammoniumchloride solutions above about 23%, limiting the usefulness of theprocess disclosed in Myerson '596. Further, contaminates of the zincoxide must be removed by an additional washing step.

U.S. Pat. No. 5,759,503 to Myerson, et al., commonly assigned with thepresent application, discloses a method for the recovery of zinc oxideby dissolving zinc oxide in an intermediate, diluting the intermediateby a factor of 3 to 30 by adding 70-100° C. water, and filtering out theresultant zinc oxide crystals. Myerson '503, along with its family ofpatents, discloses using ammonium chloride solutions of 23% and teachthat using higher concentration ammonium chloride solutions will produceundesired results. This was the understanding at the time of inventionof the processes disclosed and claimed in the Myerson '503 patent andits family of patents. These undesired results include the precipitationof various zinc species, resulting in crystallization of some species.Because of this, using the cooling step disclosed in Myerson '596, onecannot use concentrations of ammonium chloride solutions above about23%, limiting the usefulness of the process disclosed in Myerson '503.Although the processes disclosed and claimed in Myerson '503 patent arevaluable and perform admirably, it has now been discovered that byaltering the process steps, including the addition of a dilution step,ammonium chloride solutions of between 23% and 30%, and of 30% orgreater, can be used to recycle industrial waste streams.

Further, Myerson '503 differs from the present invention in thatconcentrations above 23% are not disclosed and, in fact, werespecifically discouraged because, at that time and using those processsteps, using ammonium concentrations above 23% did not result in asuitable process because concentrations of ammonium chloride above about23% tend to precipitate out ammonium chloride along with the zinc oxidewhen the solution is cooled. It is undesirable to have the ammoniumchloride precipitate out. This problem is solved by the presentinvention.

In general, the Myerson family of patents specifically discloses methodsfor the recovery of zinc oxide comprising the steps of treating a wastestream with a 23% ammonium chloride solution and separating undissolvedcomponents from the solution. It is further taught that NH₄Clconcentrations greater than 23% result in an impure zinc oxide due tothe tendency of the NH₄Cl to precipitate out of solution with the zinccompounds at such high concentrations. Furthermore, the cooling of theproduct solution results in the precipitation of various zinc species,resulting in the unwanted crystallization of some species. Because ofthis, prior to the present invention, the inventor and those skilled inthe art were under the belief that one could not use concentrations ofammonium chloride solutions above about 23%, thus limiting theusefulness of the process disclosed in the prior Myerson patents.

Therefore, there exists a need for an alternative method that willrecover essentially pure zinc oxide from industrial waste materials thatis economical, quick, and efficient and, optionally also will allow therecovery of elemental lead, cadmium, and copper from industrial wastematerials, at the lower end of the temperature range and at higher leachsolution concentrations than previously thought possible.

BRIEF SUMMARY OF THE INVENTION

The present invention satisfies these needs in a method that recoversconstituents contained in common industrial waste streams from, forexample, the metals industries. Many of these industrial waste streamscontain zinc, lead, copper, silver, and/or cadmium compounds, all ofwhich can have a commercial value, or which are undesirable wasteproducts. For example, one commercially valuable material that can berecovered is essentially pure zinc oxide from industrial waste materialstreams containing zinc or zinc oxide. Along with the essentially purezinc oxide, zinc metal also can be recovered, as well as values of othermetallic elements originally contained in the waste material, such aslead, silver, and cadmium. The solutions used in the process arerecycled such that liquid wastes are kept to a minimum and, ideally,eliminated. The solids recovered from the process, namely, the chemicaland metal values, and other residues all can be used in other processes.One such residue, an iron oxide cake, is of such a quality that it canbe used directly as the feedstock for the typical steel productionprocess.

Briefly, the present invention provides a method for recovery of highpurity chemical and/or metal values from industrial waste streams suchas electric arc furnace effluents (dusts, fumes, and vapors) containingthe values or compounds containing the values (such as zinc compounds)comprising the steps of: (a) leaching the waste stream with an ammoniumchloride solution at a temperature of at least about 70° C., resultingin a first product solution and undissolved materials; (b) adding adisplacing element of compound (such as zinc metal) to the first productsolution in a cementation step, whereby displaceable ions contained inthe first product solution are displaced by the displacing element orcompound and precipitate out of the first product solution, generally asmetals, leaving a second product solution; and (c) diluting the secondproduct solution with water, resulting in the precipitation of thedesired high purity value (such as zinc oxide) and a third productsolution. The dilution step circumvents the crystallization of variousspecies, allowing the use of the higher concentration ammonium chloridesolution of approximately 23% or greater, and preferably 30% or greater,by weight. The various undissolved precipitates produced during theprocess comprise both waste products and chemical and metal values thatcan be recovered and sold, used in subsequent processes, or added to thefeed to various industrial processes such as the iron and steel makingprocesses.

The third product solution is concentrated after removing the desiredhigh purity value (such as zinc oxide), resulting in a fourth productsolution comprising greater than 30% ammonium chloride. The fourthproduct solution then is combined with the original ammonium chloridesolution of Step (a) to leach the industrial waste stream in acontinuous process.

If the industrial waste material stream contains significant amounts ofiron, the waste stream preferably is heated in a reducing atmosphereprior to leaching, resulting in an iron-containing residue and acombustion product (dust, fumes, and/or vapors) waste stream comprisingzinc oxide. The dust, fumes and/or vapors then are subjected to theprocess disclosed in this specification.

In an extension of the present invention, using a waste stream such astypical electric arc furnace dust (which typically contains zinc,cadmium, copper, lead, and iron metals and compounds) the precipitatedmetals will comprise zinc, lead, cadmium and copper compounds. Thisalternate embodiment further comprises the additional steps of: (1)treating the precipitated metals with an aqueous solution of eitherH₂SO₄ or NH₄SO₄, whereby zinc, cadmium, and copper compounds go intosolution and lead compounds do not, resulting in a fifth productsolution comprising zinc, cadmium and copper compounds and a secondundissolved precipitate comprising lead compounds; (2) adding zinc metalto the fifth product solution, whereby cadmium and copper compounds aredisplaced by the zinc metal and precipitate out of the fifth productsolution as a third precipitate, leaving a sixth product solution; and(3) treating the sixth product solution with calcium chloride, resultingin the precipitation of CaSO₄ from the sixth product solution. Again,the undissolved precipitates produced during the process comprise bothwaste products and chemical and metal values that can be recovered andsold, used in subsequent processes, or added to the feed to variousindustrial processes such as the iron and steel making processes.

If it is desired to control sodium or potassium chloride concentrationsduring the process, additional steps of: (i) adjusting the pH of thesecond product solution to between about 5 and about 8 prior to step(c);(ii) cooling at least a portion of the second product solution toprecipitate diamino zinc dichloride; (iii) evaporating the secondproduct solution to precipitate NaCl and KCl; and (iv) combining thesecond product solution with the ammonium chloride solution to leach thewaste stream. Typically, the second product solution is acidic and abase is added to adjust the pH of the second product solution. However,if the second product solution is basic, an acid is added to adjust thepH of the second product solution. In any event, a base, an acid or acombination can be added to the second product solution to achieve thedesired pH of between 5 and 8. Suitable bases include NH₄OH, NaOH, KOH,and Ca(OH)₂. Suitable acids include hydrochloric acid, acetic acid andnitric acid.

Therefore, it is an object of the present invention to provide a methodfor recovering chemical and/or metal values from industrial wastematerial streams, such as fly ash or flue dust, which contain othermetals and/or compounds, such as iron oxide, zinc, zinc oxide, leadoxide, cadmium, copper and other materials.

It is another object of the present invention to provide a method forrecovering zinc oxide from waste materials, such as fly ash or fluedust, which contain other metals, such as iron oxide, lead oxide,cadmium, copper and other materials.

Yet another object of the present invention is to provide a method forrecovering zinc oxide in which all leaching and washing solutions arerecycled for further use, and no leaching or washing solutions aredisposed of into the sewers or the environment.

Still another object of the present invention is to provide a method forrecovering zinc oxide that also results in the precipitation inelemental form of any lead, cadmium and copper metals contained in thestarting materials.

Another object of the present invention is to provide a method forrecovering zinc metal, zinc oxide and/or iron oxide that is economical,quick and efficient.

These objects and other objects, features and advantages of the presentinvention will become apparent to one skilled in the art when thefollowing Detailed Description of the Preferred Embodiments is read inconjunction with the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of the purification steps executed in thepreferred embodiment of the present invention.

FIG. 2 is a flow chart of the purification steps executed with theoptional reduction step, in accordance with an alternative embodiment ofthe present invention.

FIG. 3 is a flow chart of the purification steps executed to furtherisolate precipitated metals, in accordance with an alternativeembodiment of the presented invention.

FIG. 4 is a flow chart of the purification steps executed to controlsodium or potassium chloride levels, in accordance with an alternativeembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a preferred embodiment of the process is shown. Thepreferred waste materials such as fly ash or flue dust from electric arcfurnaces, as well as other waste materials streams, including othermetal producing and altering processes, contain economically valuableamounts of metal and chemical values, including but not limited to zinccompounds and oxides of iron, lead, calcium, potassium, cadmium, andcopper. The preferred waste is dust, solids, vapors and/or fumes from asteel making process.

In step 100, a waste material is leached in an ammonium chloridesolution resulting in a first product solution and undissolvedmaterials. The ammonium chloride solution is greater than 23% ammoniumchloride, and preferably 30% or greater ammonium chloride, in water at atemperature above about 70° C. Prior art concentrations were not above23% due to the tendency of ammonium chloride to precipitate out withzinc oxide at concentrations above 23%. Likewise, the crystallizationstep used in prior art methods also would not operate properly atammonium chloride concentrations above 23% due to the transformation ofzinc complexes with very low zinc composition such as the tetra aminozinc complex. As explained in more detail below, the present process hasovercome such limitations. The majority of the waste materials,including any zinc and/or zinc oxide, lead oxide, cadmium oxide, copperoxide, and other metal oxides, dissolves in the ammonium chloridesolution forming a first product solution. Iron oxide, also typicallypresent in the preferred waste materials, does not dissolve in theammonium chloride solution, and remains as an undissolved material 102.Any other constituents in the waste materials not soluble in ammoniumchloride also remain as undissolved materials 102. The product solutionis filtered and the undissolved materials 102 are removed 102.

If iron oxide is present in the waste materials, the undissolvedmaterials 102 separated from the first product solution typically isrich in iron oxide, and also typically has some impurities such as zincferrite. The undissolved materials 102 can be used as a feedstock forsteel mills so long as the quantity of impurities is not too great. Inany event, it is preferable to remove the impurities from the iron oxideprior to using the iron oxide as a feedstock. Reducing the iron oxide inthe undissolved materials 102 to direct-reduced iron (DRI) also isdesired as DRI can be used to replace part or all of the steel scrapcharge. This is a first example of how even the waste from the presentprocess is usable, resulting in an environmentally friendly process.

The iron oxide in the undissolved materials 102 can be reduced to DRI intwo manners. First, carbon, in the form of activated carbon, carbondust, carbon pellets or the like, can be introduced to the ammoniumchloride and waste material mixture during the leaching process. Thecarbon reduces the iron oxide resulting in DRI upon heating. Second, thecarbon can be introduced to the dried undissolved material cake using aribbon blender. The carbon will react with the iron oxide, reducing theiron oxide to DRI upon heating. Adding heat to this process assists inthe reduction.

The remaining first product solution contains zinc oxide and otherdissolved metal oxides such as oxides of lead, cadmium, and copper.While this first product solution is still hot, finely powdered zincmetal is added to the first product solution in a cementation step 104.Through an electrochemical reaction, any lead, cadmium, and copper metalin solution plates out onto the surfaces of the zinc metal particles.The addition of sufficient powdered zinc metal results in the removal ofmost of the lead, cadmium, and copper of the first product solution byprecipitation. After cementation, there is now a second product solutionand a precipitate 108.

The second product solution is rich in zinc compounds, while theprecipitate 108 is rich in oxides of lead, cadmium, and copper. Silvercompounds may be found in the precipitate and/or the second productsolution. The precipitate 108 is separated from the second productsolution and, as will be described in more detail below, can optionallybe treated further to recover the economically valuable metals. This isa second example of how the waste from the present process is usable,reducing the amount of waste form the process.

Purified zinc may be recovered as either zinc oxide by diluting thesecond product solution in a dilution step 106 or as elemental zinc byelectrowinning (not shown). Recovery of zinc oxide by dilution ispreferred, as it has been discovered that this allows the use of higherconcentrations of ammonium chloride solutions of greater than 23%, andpreferably 30% or greater, by weight. To recover zinc oxide in thedilution step 106, the second product solution is diluted to aconcentration between about 2% and about 12% ammonium chloride. As theconcentration of ammonium chloride in the second product solution drops,the solubility of zinc oxide decreases, thereby resulting in theprecipitation of zinc oxide from the second product solution. Whendiluting the second product solution, it is preferable to add the secondproduct solution to the water, rather than adding water to the secondproduct solution. The resultant zinc oxide has significantly lowerchloride content if the second product solution is added to the water.After dilution, there is now a third product solution and precipitatedzinc oxide 112.

As disclosed above, the preferred method of dilution involves adding thesecond product solution to the water, and not adding the water to thesecond product solution. Preferably, the water should be at 60° C. orhigher during the entire dilution step. The second product solutionshould be added to the water in a gradual fashion. For example, in abatch system, one-third of the second product solution should be addedto the total required amount of water and stirred to form a firstintermediate solution. The remainder of the second product solution,either in one large batch or in two or more smaller batches, should thenbe added to the first intermediate solution to form the third productsolution. This allows the ammonium concentration to be lowered to arelatively low level in the first intermediate solution, forcing thedilution to proceed along a defined path. The zinc oxide precipitatesout of the first intermediate solution such that the concentration ofammonium chloride in the first intermediate solution is even lower whenthe remainder of the ammonium chloride solution is added to the firstintermediate solution. Staged dilution with as few as two steps and asmany as ten steps or more can be used, depending on the specificcircumstances, such as, but not limited to, composition and quantity ofwaste stream.

In another example, in a continuous system, a first fraction of thesecond product solution is added to water in a first vessel to form thefirst intermediate solution. The first intermediate solution should havea concentration of ZnO/NH₄Cl to water of between about 0.2% and 6%,preferably about 3.3%. Any zinc oxide precipitating out of the firstintermediate solution is removed from the first vessel. The first vesselis maintained at steady state at the desired concentration by removingany zinc oxide precipitate, adding additional water or second productsolution, and/or removing first intermediate solution. The removed firstintermediate solution is transferred to a second vessel where a secondfraction of the second product solution is added to the firstintermediate solution to form a second intermediate solution. The secondintermediate solution should have a concentration of ZnO/NH₄Cl to waterof between about 2% and 9%, preferably 6.6%. The second vessel ismaintained at steady state at the desired concentration by removing anyzinc oxide precipitate, adding additional second product solution,and/or removing second intermediate solution. The removed secondintermediate solution is transferred to a third vessel where a thirdfraction of the second product solution is added to the secondintermediate solution to form a third intermediate or final solution.The third intermediate or final solution should have a concentration ofZnO/NH₄Cl to water of between about 5% and 12%, preferably 10%. Thisalso allows the ammonium concentration to be lowered to a relatively lowlevel in the first intermediate solution, forcing the dilution toproceed along a defined path. The zinc oxide precipitates out of thefirst intermediate solution such that the concentration of ammoniumchloride in the first intermediate solution is even lower when theremainder of the ammonium chloride solution is added to the firstintermediate solution.

The use of thirds as the fractions of second solution added to eachvessel is preferred, but it should be understood that any fraction ofthe second product solution can be added to the total required amount ofwater at a time. The important criteria is that the amount of the secondproduct solution added to the total required amount of water be kept toa level allowing for the greatest amount of zinc oxide to precipitateout of the first intermediate solution. This also prevents the formationof tetra-amino-zinc-chloride, a low zinc content, fine needle crystalthat is difficult to filter separate or wash. The amount of waternecessary, and the addition rates of second product solution to thewater, can be determined by one of ordinary skill in the basic chemicalarts without undue experimentation. Further, determining the totalrequired amount of water is simple chemistry and well within the skillof the person of ordinary skill in the art of this invention. It simplyis enough water to lower the concentration of ammonium chloride in thesecond product solution, which can be determined by simple chemicalanalysis, to a theoretical concentration of from about 2% to about 12%ammonium chloride.

The third product solution is filtered to remove the precipitated zincoxide 112, which is placed in a drying oven at a temperature of over100° C. After a sufficient drying period, the resultant dry white powderis essentially pure zinc oxide.

If it is desired to recover elemental zinc, the second product solutionis subjected to electrolysis in an electrolytic cell containing an anodeand a cathode. The second product solution comprises zinc ions insolution as Zn²⁺, which will be electrodeposited on the cathode.Although it is preferable to have the cathode made from zinc metal,cathodes of other materials will allow the electrodeposition of zincmetal from the second product solution.

Any of the electrolysis cells discussed in the literature are suitable,as long as such cells are configured for the electrolysis of zinc ioncontaining solutions. The two electrodes of the electrolysis cells areconnected externally to a power supply capable of impressing a suitablevoltage across the electrodes. The zinc ions, being positive in nature,migrate toward the negative electrode, or cathode, where they combinewith electrons supplied by the external circuit to form neutral zincmetal atoms. When this happens, the zinc metal, in effect, electroplatesonto the cathode. By using a zinc cathode, the entire cathode can beremoved and used as necessary as a source of zinc. Alternatively, acathode on which electroplated zinc metal can be easily removed can beused.

If zinc oxide is precipitated from the second product solution bydilution, the diluted third product solution contains ammonium chlorideand other compounds. Rather than dispose of this solution, it ispreferable to produce a more concentrated (>30%) ammonium chloridesolution 110 that is recycled back to the leaching step 100. This can beaccomplished using evaporators or reverse osmosis membrane technology.This is a third example of how the waste from the present process isusable, reducing the amount of waste produced by the present process toa minimum.

From an economically competitive situation, the use of reverse osmosismembrane technology to concentrate the third product solution to obtaina concentrated ammonium chloride solution on one side of the membranewill save feed costs. Every so often, it will be necessary to back flushthe salts off the membrane to recover them for makeup use in the future.In essence, reverse osmosis membrane technology is using a pump to pumpa solution through a membrane, which is significantly lower in cost thanburning natural gas in an evaporator condenser to evaporate water from asolution. This technology is used to filter out sodium chloride and theminerals out of seawater to make distilled water.

Referring to now FIG. 2, if there is significant iron content in thewaste material, it is preferable to first heat the waste material in areducing atmosphere in a reduction step 200, prior to the leaching step100. The waste material is typically reduced at a temperature greaterthan 420° C., and preferably between 700° C. and 1300° C. The reducingatmosphere can be created by using hydrogen gas, simple carbon speciesgases, such a carbon dioxide, or by heating the material in an oxygencontaining gas in the presence of elemental carbon. The carbon, whichmay be in the form of powdered coke or coal, is mixed with the iron andzinc containing waste prior to the reduction step. Examples of reductionprocesses include rotary hearth and rotary kiln furnaces. In theseprocesses, the powdered mixture is formed into briquettes or pelletsbefore feeding to the furnace. The iron oxide is reduced to metalliciron and remains within the briquette or pellet. Zinc, lead and cadmiumare reduced and leave the briquette or pellet in vapor form that issubsequently oxidized on contact with oxygen. Alternatively, a mixturecontaining electric arc furnace dust may be added back to the electricarc furnace that operates under reducing conditions. Mixtures of wastematerials such as electric arc furnace dust, oily mill scale and otherflue dusts can be used to improve the value of the iron product. Thecarbon is preferably in the form of dust or pellets. Typically heatingtimes are from 4 minutes to 4 hours, depending on the method used.

The heating and reduction step 200 results in the reduction of the ironcompounds in the waste material into direct reduced iron (DRI), and thegeneration of volatile materials and combustion products. The DRI can befed directly into a steel mill as a feed source. The combustionproducts, in the form of exhaust dusts, vapors, and fumes, are recoveredby a filter process, such as a bag house or a wet scrubber. The zinc,lead, cadmium, and copper in the waster material are vaporized in theheating and reduction step 200 and comprise a majority of the exhaustdusts, vapors, and fumes. The heating and reduction step 200 is notnecessary if there is no significant iron content in the waste. Theheating and reduction step 200 is a fourth example of how waste from thepresent process (in this case iron oxides) is usable, further reducingthe amount of waste produced by the present process.

Once the waste material is reduced, the process as represented in FIG. 2continues just as the preferred embodiment represented in FIG. 1. Thecombustion product (the exhaust dusts, vapors, and fumes) are leached inleaching step 100, undissolved materials 102 are separated from theleachant, and the leachant (first product solution) is subjected tocementation step 104 to remove Pb, Cd, and Cu, for further processing108. The remaining solution (second product solution) is diluted withwater in a dilution step 106 to 2-12% NH₄Cl, thus prompting ZnO toprecipitate out of the solution. The now remaining solution (thirdproduct solution) is then concentrated back to >30% NH₄Cl in aconcentration step 110 and is recycled back into the process as aleaching solution.

Waste materials typically have varying amounts of lead, cadmium, andcopper metals contained therein. For various reasons, it is desirable toremove such metals from the waste materials, for example, to recycle thelead, cadmium, and copper, or to prevent their release into theenvironment. Referring to FIG. 3, the lead, cadmium, and coppercompounds removed from the first product solution during the cementationstep 104 as precipitates 108 can be recovered by first treating thecompounds with H₂SO₄ or (NH₄)₂SO₄ in a dissolution step 300. The zinc,cadmium, and copper compounds will go into solution while the leadcompounds will not. The lead compounds are filtered out 304, leaving asolution of zinc, cadmium and copper compounds 302.

The Zn, Cd, and Cu—containing solution 302 then is treated with zincpowder in a second cementation step 306 to result in theelectrodeposition of Zn and Cd on the zinc, which then is filtered out310, leaving a solution of zinc compounds 308. To this solution, calciumchloride is added. The calcium reacts with the sulfate present in thesolution from the H₂SO₄ or (NH₄)₂SO₄ to form CaSO₄, when precipitatesout. The CaSO₄ is filtered out 314 and the remaining solution can berecycled back to the leaching step to further recover zinc 312.

Referring to FIG. 4, sodium and potassium chlorides, introduced by theinitial waste materials stream, may accumulate in the various productsolutions, thus adversely affecting process performance. The chloridelevel may optionally be controlled by periodic removal of chloridesalts. This is preferably accomplished by adjusting the pH of the secondproduct solution in a pH adjustment step 400 prior to the dilution step106. This will neutralize the acidic effects of the chloride ions byforming reacting therewith to form ammonium chloride and water.

The second product solution typically is acidic, with a pH of less than5. Thus, the pH of the typical second product solution is adjustedupwards using a base. Preferred bases include NH₄OH, NaOH, KOH, andCa(OH)₂. However, it is possible that the second product solution has apH of between 5 and 8. If so, the pH of the second product solution canbe adjusted upwards or downwards within the 5 to 8 range using either anacid or base, or both. Preferred acids include HCl, acetic acid, andHNO₃. In the event the second product solution has a pH of greater than8, the pH of the second product solution is adjusted downwards using anacid.

At least a portion of the pH-adjusted second product solution may beremoved through a purge stream prior to the dilution step 106 andallowed to cool in a cooling step 404. As the purged pH-adjusted secondproduct solution cools, a portion of the zinc compounds will precipitateout of the purged pH-adjusted solution as diamino zinc dichloride, whichis then filtered out in a filtration step 408. The use of a purge streamassists in the control of the concentration of sodium chloride and/orpotassium chloride in the second product solution. It is preferred tokeep the concentrations of sodium chloride and potassium chloride downand if these concentrations build up, a larger purge stream is used.

The remaining product solution (third product solution) is evaporated orotherwise concentrated in concentration step 110 to precipitate sodiumchloride and potassium chloride, which are filtered out in a secondfiltration step 410. The remaining concentrated solution can then bereintroduced into the process at the leaching step 100. In this way,chloride levels are maintained as to not adversely effect the formationof substantially pure zinc oxide.

The above-disclosed process can be used in connection with any wastestream or other material having a suitable percentage of zinc. Zinccontaining ash materials from a number of industries that are capturedin pollution control systems also are suitable for this processincluding, but not limited to, integrated steel industry processes,galvanizing processes, iron foundries, and brass foundries.

The above description sets forth the best mode of the invention as knownto the inventor at this time, and is for illustrative purposes only, asit is obvious to one skilled in the art to make modifications to thisprocess without departing from the spirit and scope of the invention andits equivalents as set forth in the appended claims.

What is claimed is:
 1. A method for recycling industrial waste streamscontaining zinc compounds comprising the steps of: (a) leaching saidwaste stream with a solution of at least 30% by weight ammoniumchloride, resulting in a first product solution and undissolvedmaterials; (b) adding zinc metal to said first product solution, wherebyzinc displaceable metal ions contained in said first product solutionare displaced by said zinc metal and precipitate out of said firstproduct solution as metals, leaving a second product solution; (c)adjusting the pH of said second product solution to a selected pH ofbetween about 5 and about 8; and then (d) diluting said second productsolution with water, resulting in the precipitation of zinc oxide and athird product solution.
 2. The method characterized in claim 1, whereinsaid solution of ammonium chloride is maintained at a temperature aboveabout 70° C.
 3. The method characterized in claim 2, wherein saidsolution of ammonium chloride is maintained at a temperature of betweenabout 70° C. and 100° C.
 4. The method characterized in claim 1, whereinthe pH of said second product solution is adjusted with at least onecompound selected from the group consisting of acids and bases.
 5. Themethod characterized in claim 4, wherein said acids are selected fromthe group consisting of hydrochloric acid, acetic acid, and nitric acid.6. The method characterized in claim 4, wherein said bases are selectedfrom the group consisting of NH₄OH, NaOH, KOH, and Ca(OH)₂.
 7. Themethod characterized in claim 1, wherein the pH of said second productsolution is adjusted with at least one compound selected from the groupconsisting of hydrochloric acid, acetic acid, nitric acid, NH₄OH, NaOH,KOH, and Ca(OH)₂.
 8. The method characterized in claim 2, wherein saidsecond product solution is diluted to between about 2% and about 12%ammonium chloride in step (d).
 9. The method characterized in claim 8,wherein said third product solution is concentrated after removing saidzinc oxide, resulting in a fourth product solution comprising greaterthan 30% ammonium chloride.
 10. The method characterized in claim 9,wherein said fourth product solution is combined with the ammoniumchloride solution of step (a) to leach said waste stream.
 11. The methodcharacterized in claim 1, wherein said waste stream is heated in areducing atmosphere prior to leaching, resulting in an iron-containingresidue and a waste stream comprising oxides of zinc.
 12. The methodcharacterized in claim 1, wherein said waste stream further compriseslead, cadmium, and copper compounds.
 13. The method characterized inclaim 12, wherein said precipitated metals comprises zinc, lead, cadmiumand copper.
 14. The method characterized in claim 13, further comprisingthe step of treating said precipitated metals with an aqueous solutionof a compound selected from the group consisting of H₂SO₄ and (NH₄)₂SO₄,whereby zinc, cadmium, and copper compounds go into solution and leadcompounds do not, resulting in a fifth product solution comprising zinc,cadmium and copper compounds and a second undissolved precipitatecomprising lead compounds.
 15. The method characterized in claim 14,further comprising the step of adding zinc metal to said fifth productsolution, whereby cadmium and copper compounds are displaced by saidzinc metal and precipitate out of said fifth product solution as a thirdprecipitate, leaving a sixth product solution.
 16. The methodcharacterized in claim 15, further comprising the step of treating saidsixth product solution with calcium chloride, resulting in theprecipitation of CaSO₄ from said sixth product solution.
 17. The methodcharacterized in claim 16, wherein said sixth product solution iscombined with said solution of ammonium chloride to leach said wastestream in step (a).
 18. The method characterized in claim 17, whereinsaid aqueous solution is H₂SO₄.
 19. The method characterized in claim17, wherein said aqueous solution is (NH₄)₂SO₄.
 20. The methodcharacterized in claim 1, further comprising the steps of: (i) coolingat least a portion of said second product solution to precipitatediamino zinc dichloride; (ii) removing said diamino zinc dichloride fromsaid second product solution; (iii) evaporating said second productsolution to precipitate NaCl and KCl; (iv) removing said NaCl and KClfrom said second product solution; and (v) combining said second productsolution with said ammonium chloride solution to leach said wastestream.
 21. A method for recycling industrial waste streams containingzinc compounds comprising the steps of: (a) leaching said waste streamwith a solution of 23% or greater by weight ammonium chloride maintainedat a temperature above 70° C., resulting in a first product solution andundissolved materials; (b) adding zinc metal to said first productsolution, whereby zinc-displaceable metal ions contained in said firstproduct solution are displaced by said zinc metal and precipitate out ofsaid first product solution as metals, leaving a second productsolution; (c) adjusting the pH of said second product solution tobetween 5 and 8; and then (d) diluting said second product solution withwater to between 2% and 12% ammonium chloride, resulting in theprecipitation of zinc oxide and a third product solution.
 22. The methodcharacterized in claim 21, further comprising the step of treating saidprecipitated metals with an aqueous solution of a compound selected fromthe group consisting of H₂SO₄ and (NH₄)₂SO₄, whereby zinc, cadmium, andcopper compounds go into solution and lead compounds do not, resultingin a recycle product solution comprising zinc, cadmium and coppercompounds and a second undissolved precipitate comprising leadcompounds.
 23. The method characterized in claim 21, wherein saidindustrial waste stream is heated in a reducing atmosphere prior toleaching, resulting in an iron-containing residue and a waste streamcomprising oxides of zinc.
 24. The method characterized in claim 21,further comprising: (e) washing the zinc oxide precipitate in water at atemperature of at least 70° C.
 25. The method characterized in claim 21,wherein the method is a continuous process.
 26. A method for reclaimingconstituents from waste streams comprising the steps of: (a) leachingsaid waste stream with a solution of at least 23% by weight ammoniumchloride maintained at a temperature above 70° C., resulting in a firstproduct solution comprising at least a portion of the constituents andundissolved materials; (b) adding a reactive compound to said firstproduct solution, whereby at least a portion of any ions contained insaid first product solution that are displaceable by said reactivecompound are displaced by said reactive compound and precipitate out ofsaid first product solution, leaving a second product solutioncomprising at least a portion of the constituents; (c) adjusting the pHof said second product solution to a selected pH of between 5 and 8; andthen (d) diluting said second product solution with water to between 2%and 12% ammonium chloride, resulting in the precipitation of at least aportion of the constituents and a third product solution.
 27. The methodcharacterized in claim 26, wherein said ammonium chloride solution has aconcentration of at least 30% by weight.
 28. The method characterized inclaim 26, wherein said waste stream comprises zinc compounds.
 29. Themethod characterized in claim 28, wherein said waste stream furthercomprises at least one additional compound selected from the groupconsisting of lead compounds, cadmium compounds, and copper compounds.30. The method characterized in claim 26, wherein said solution ofammonium chloride is maintained at a temperature of between about 70° C.and 100° C.
 31. The method characterized in claim 26, wherein the pH ofsaid second product solution is adjusted with at least one compoundselected from the group consisting of acids and bases.
 32. The methodcharacterized in claim 31, wherein said acids are selected from thegroup consisting of hydrochloric acid, acetic acid, and nitric acid. 33.The method characterized in claim 31, wherein said bases are selectedfrom the group consisting of NH₄OH, NaOH, KOH, and Ca(OH)₂.
 34. Themethod characterized in claim 26, wherein the pH of said second productsolution is adjusted with at least one compound selected from the groupconsisting of hydrochloric acid, acetic acid, nitric acid, NH₄OH, NaOH,KOH, and Ca(OH)₂.